Current developments of microelectronics and optoelectronics industries have made higher requirements on quality of nitrides, especially gallium nitride (GaN) and silicon nitride (Si3N4). GaN is regarded as the basis of new-generation light-emitting diode (LED). GaN epitaxial wafers, which may be used for manufacturing blue and green LEDs and blue light lasers, are currently prepared by reacting trimethyl Gallium (Ga(CH3)3) with high purity ammonia (NH3) under high temperature in MOCVD (Metal Organic Chemical Vapor Deposition). As an insulating material, Si3N4 is widely used in fields of for example, ICs (Integrated Circuits), LCD (liquid crystal display) and solar cells. Si3N4 is deposited by reacting the high purity NH3 with SiH4. In addition, the high purity ammonia is widely used in fields of super-hard ceramics (e.g. BN), biology and pharmaceutics.
In the synthesis of top-quality nitrides, high purity ammonia has to be used, as the purity of ammonia has a great impact upon the above industries. In the preparation of silicon nitride, for instance, instead of Si3N4, silicon oxide (SiO2) will be formed if as little as 50 ppm of water or oxygen appears in the ammonia used. Also for example, in GaN MOCVD, light-emitting wavelength of resulted epitaxial wafers will be uncontrollable if the ammonia used contains 3 ppm of water or oxygen. For development of optoelectronics and microelectronics industries, the development of basic raw materials represented by high purity ammonia becomes extremely important.
In order to obtain high quality nitrides used in micro-electronics industry, the purity requirement on ammonia should be very high. The following table shows the typical indexes of high purity ammonia currently used and boiling points of some common impurities.
TABLE 1Typical Indexes of Gaseous 6N and 7N Ammonia (ppb)NH3Ar + O2CO2COCH4H2OTotal6N (99.9999%)<120<100<50<50<200<10007N (99.99999%)<20<50<10<10<25<100
TABLE 2Boiling Points of Ammonia and some Common ImpuritiesGasH2N2ArO2CH4CO2NH3H2OBoiling Point20.2777.3587.590.2116.7195.2239.8373.2(K)
Molecular weights of these substances do not differ greatly, in which case, the boiling point of each substance usually reflects its molecular interactions. For the polar molecules such as water and ammonia, the stronger the molecular interaction is, the higher the boiling point is; while for the non-polar molecules such as H2 and N2, the weaker the molecular interaction is, the lower the boiling point is. In the case of the non-polar molecules, the smaller the molecular weight is, generally the lower the boiling point is. For instance, boiling point of H2 is lower than those of N2 and O2. However, there are some exceptions like CH4, which is of smaller molecular weight compared with N2 and O2, yet of higher boiling point, since the molecule of CH4 is composed of 5 atoms, and has a larger molecular volume, hence has a stronger molecular interaction.
There is a wide difference between the polar molecule and the non-polar molecule in the strength of the molecular interactions, which is reflected not only in the boiling points, but also in the gas purification processes, i.e. it is more difficult to purify polar molecular gases (e.g. NH3) than non-polar molecular gases (e.g. H2 and N2). If the purification of polar molecular gas (e.g. NH3) can be converted to the purification of non-polar molecular gas (e.g. H2 and N2), it will be easier to obtain the polar molecular gas of high purity.
It's shown in the above table that the boiling point of ammonia is neither the highest nor the lowest among ammonia and its impurities. Therefore, to purify the ammonia, a two-step process has to be used in the art, in which low boiling point impurities like O2 and CO2 are firstly removed in an adsorption step, and then substances with boiling point higher than that of ammonia, such as H2O, are removed in a distillation or adsorption step.
The most arduous task in the technology for obtaining high purity ammonia lies in elimination of water in the ammonia. Since both NH3 and H2O are polar molecules, they are bound together by hydrogen bonds. Because of the saddle-shaped purification curve for water in the purification of NH3, it is difficult to obtain the ammonia with the content of H2O lower than 1 ppm. Therefore, to remove trace of water from NH3 by common processes is extremely difficult. Almost all technologies and patents aimed at obtaining high purity ammonia are focused on the elimination of water.
Currently the most popular method for preparation of the high purity ammonia is purifying the low purity ammonia by first obtaining ordinary purity ammonia by synthesizing ordinary purity N2 and H2, and then getting high purity ammonia through purification, as is detailed in patent applications ‘On-site generation, purification, and distribution of ultra-pure anhydrous ammonia’ (Matheson Tri-gas Co., Ltd. Chinese patent application No. 200580022983.8) and ‘Process for purifying ammonia’ (JAPAN PIONICS, Chinese patent application No.01124353.8).
In preparing high purity ammonia by using ammonia purification method, the kinds and contents of impurities vary with the sources of the ammonia, which are generally as listed below: oil, ˜2000 ppm; N2, ˜2000 ppm; H2, ˜2000 ppm; H2O, 300˜600 ppm; CH4, 600˜800 ppm; O2+Ar, 50˜200 ppm. For the high purity ammonia used in microelectronics, H2O, O2 and oil are the most harmful substances, which must be removed to ensure that their contents meet the requirements.
While the price of ordinary ammonia is quite low, high purity ammonia is very expensive. It is very difficult to obtain high purity ammonia by purifying ordinary ammonia because the bonding between the impurities and ammonia is so strong that it is hard for them to be separated physically or chemically. Among all these impurities, oxygen-containing impurities, primarily O2 and H2O, especially H2O, are the most difficult impurities to remove, while contents of oxygen-containing impurities are the most important indexes indicating the quality of high purity ammonia.
In brief, there are many kinds of impurities in ordinary ammonia, especially oxygen-containing impurities, which are very difficult to remove, hence the ammonia purification is rather difficult and the price of high purity ammonia is extremely high.